noun. A procedure that measures minute amounts of a substance, such as a hormone or drug, by quantitating the binding, or the inhibition of binding, of a radiolabeled substance to an antibody.

noun. A procedure that uses the binding of a radioactively labeled substance to an antibody in order to analyze minute amounts of biological compounds

noun. immunoassay of a substance that has been radioactively labeled

"She and a younger colleague, Solomon Berson, developed methods for the radioimmunoassay -- an analytic tool that allows investigators to survey minute quantities of hormones, viruses, and chemicals in human beings."

immunoassay  immunochemical assay